This invention applies to video or graphics projection systems that use color panels having a resolution of H.times.V pixels, where source images or sequences are available at higher resolutions, e. g., 2H.times.2V, or greater. The commonly known methods for displaying images with higher resolution than the individual display panels resolution include the following:
1) Direct subsampling without filtering of the high resolution image to the lower panel resolution;
2) Filtering or other local spatial averaging prior to subsampling down to the resolution in order to prevent aliasing;
3) Subsampling, with or without filtering, down to the resolution and applying spatial image enhancement techniques such as unsharp masking or high-pass filtering to improve the perceived appearance of the displayed image.
In all three of the known techniques, there is a loss of spatial information from the high resolution image. Technique 1 tends to preserve sharpness but also causes aliasing to occur in the image. Technique 2 tends to prevent aliasing but results in a more blurred image. Technique 3 can result in an image that has little or no aliasing and can appear sharper by using high-pass filtering which steepens the slope of edges. However, technique 3 has limitations in that overshoots result on the edges, causing "haloing" artifacts in the image. Also, because technique 3 has no further true image information than techniques 1 or 2, there is a general loss of low-amplitude, high-frequency information, which is necessary for true rendition of textures. The effect on textures is that they are smoothed. Important low-amplitude texture regions include hair, skin, waterfalls, lawns, etc.
U.S. Pat. No. 4,484,188, "Graphics Video Resolution Improvement Apparatus," to Ott, discloses a method of forming additional video lines between existing lines and combining the data from the existing lines by interpolation. It is primarily intended for graphics character applications and the prevention of rastering artifacts, also know as "edge jaggies".
U.S. Pat. No. 4,580,160, "Color Image Sensor with Improved Resolution Having Time Delays in a Plurality of Output Lines," to Ochi, uses a 2D hexagonal element sensor array which is loaded into a horizontal shift register. Delays are used to load alternating columns into the register, thus providing an increase in resolution for a given register size.
U.S. Pat. No. 4,633,294, "Method for Reducing the Scan Line Visibility for Projection Television by Using Different Interpolation and Vertical Displacement for Each Color Signal," to Nadan, discloses a technique that spatially shifts, in the vertical, the red, green and blue (RGB) scan lines with respect each other in order to reduce the visibility of the scan lines. Interpolation of the data for the offset scan lines color plane is used to reduce edge color artifacts.
U.S. Pat. No. 4,725,881, "Method for Increasing the Resolution of a Color Television Camera with Three Mutually Shifted Solid-State Image Sensors," to Buchwald, uses spatially shifted sensors to capture the RGB image signals. The shift allows a higher resolution color signal to be formed, which is then transformed into Y, R-Y, and B-Y signals. The luminance signal is low-pass-filtered (LPF), high-pass-filtered (HPF), and the two filtered signals added together. The color signals are low-pass filtered, and further modulated by a control signal which is formed from the high-pass filtered luminance signal. The luminance signal acts as a control for modulating the amplitude of the color signals.
U.S. Pat. No. 5,124,786, "Color Signal Enhancing Circuit for Improving the Resolution of Picture Signals," to Nikoh, splits the chrominance image signals into LPF and HPF halves. The HPF half is amplified and added back to the LPF. The purpose is to boost high frequency color without affecting the luminance signal.
U.S. Pat. No. 5,398,066, "Method and Apparatus for Compression and Decompression of Digital Color Images," to Martinez-Uriegas et al., uses color multiplexing of RGB pixels to compress a single layer image. The M-plane, which is defined as a method of spatially combining different spectral samples, is described and is referred to as "color multiplexing." Methods for demultiplexing the image back to three full-resolution image planes, and the CFA interpolation problem, are discussed, as are various correction technique for the algorithms artifacts, such as speckle correction for removing 2-D high frequency chromatic regions.
U.S. Pat. No. 5,528,740, "Conversion of Higher Resolution Images for Display on a Lower-Resolution Display Device," to Hill et al., is a system for converting a high-resolution bitonal bit-map for display on a lower-resolution pixel representation display. It introduces the concept of "twixels" which are multibit pixels that carry information from a number of high-resolution bitonal pixels. This information may trigger rendering decisions at the display device to improve the appearance of text characters. It primarily relates to the field of document processing.
U.S. Pat. No. 5,541,653, "Method and Apparatus for Increasing Resolution of Digital Color Images Using Correlated Decoding," to Peters, describes a technique for improving luminance resolution of captured images from 3 CCD cameras, by spatially offsetting the RGB sensors by 1/2 pixels.
U.S. Pat. No. 5,543,819, "High Resolution Display System and Method of Using Same," to Farwell et al., uses a form of dithering to display high-resolution color signals, where resolution refers to amplitude resolution, i.e., bit-depth, on a projection system using single-bit LCD drivers.
Tyler, et al., Bit Stealing: How to get 1786 or More Grey Levels from an 8-bit Color Monitor, Proc of SPIE, V. 1666, pp 351-364, 1992, describes a display enhancement technique. It exploits the spatio-color integrative ability of the human eye in order to increase the amplitude resolution of luminance signals by splitting the luminance signal across color pixels. It is intended for visual psychophysicists studying luminance perception who need more than the usual 8-bits of greyscale resolution that are offered in affordable RGB 24-bit displays. Such studies do not require color signals, because the images displayed are grey level, and the color rendering capability of the display is thus sacrificed to create higher bit-depth grey level signals. In this case, the three color pixels contributing to the luminance signals are viewed with such a pixel size & viewing distance that the three pixels are merged into a single perceived luminance element. In other words, the pixel spacing of the three pixels causes them to be above the highest spatial frequency perceived by the visual system. This is true for luminance, as well as chromatic frequencies.